Hi,

I'm wondering if it's possible that certain physical device doesn't
handle flush correctly.

E.g. some vendor does some complex logical in their hdd controller to
skip certain flush request (but not all, obviously) to improve performance?

Do anyone see such reports?

And if proves to happened before, how do we users detect such problem?

Can we just check the flush time against the write before flush call?
E.g. write X random blocks into that device, call fsync() on it, check
the execution time. Repeat Y times, and compare the avg/std.
And change X to 2X/4X/..., repeat above check.

Thanks,
Qu